Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with pathogen, insect feeding, and parasitism by another plant such as mistletoe. Abiotic stresses include, for example, excessive or insufficient available water, nitrogen, temperature extremes, and synthetic chemicals such as herbicides.
Abiotic stresses such as drought, high salinity and deficiency of nutrient elements adversely affect the growth and productivity of plants including crops, which significantly limit crop production worldwide. Cumulatively, these factors are estimated to be responsible for an average 70% reduction in agricultural production. Plants are sessile and have to adjust to the prevailing environmental conditions of their surroundings. This has led to their development of a great plasticity in gene regulation, morphogenesis, and metabolism. Adaptation and defense strategies involve the activation of genes encoding proteins important in the acclimation or defense towards the different stressors.
Drought (insufficient available water) is one of the major abiotic stresses that limit crop productivity worldwide, and exposure of plants to a water-limiting environment during various developmental stages appears to activate various physiological and developmental changes. Although many reviews on molecular mechanisms of abiotic stress responses and genetic regulatory networks of drought stress tolerance have been published (Valliyodan, B., and Nguyen, H. T. (2006) Curr. Opin. Plant Biol. 9:189-195; Wang, W., et al. (2003) Planta 218:1-14; Vinocur, B., and Altman, A. (2005) Curr. Opin. Biotechnol. 16:123-132; Chaves, M. M., and Oliveira, M. M. (2004) J. Exp. Bot. 55:2365-2384; Shinozaki, K., et al. (2003) Curr. Opin. Plant Biol. 6:410-417; Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005) Trends Plant Sci. 10:88-94), it remains a major challenge in biology to understand the basic biochemical and molecular mechanisms for drought stress perception, signal transduction and tolerance. Genetic research has shown that drought tolerance is a quantitative trait, controlled by many genes. Molecular marker-assisted breeding has led to improved drought tolerance in crops. However, marker accuracy and breeding efficiency remain problematic (Ashraf M. (2010) Biotechnol. Adv. 28:169-183). Transgenic approaches to engineering drought tolerance in crops have made progress (Vinocur B. and Altman A. (2005) Curr. Opin. Biotechnol. 16:123-132; Lawlor D W. (2013) J. Exp. Bot. 64:83-108).
The absorption of nitrogen by plants plays an important role in their growth (Gallais et al., J. Exp. Bot. 55(396):295-306 (2004)). Plants synthesize amino acids from inorganic nitrogen in the environment. Consequently, nitrogen fertilization has been a powerful tool for increasing the yield of cultivated plants, such as rice, maize and soybean. Lack of sufficient plant available nitrogen for optimum growth and development may be considered as an abiotic stress. In order to avoid pollution by nitrates and to maintain a sufficient profit margin, today farmers desire to reduce the use of nitrogen fertilizer. If a plant variety has increased nitrogen assimilation capacity, it would also be expected to have increased growth and yield. In summary, plant varieties that have better nitrogen use efficiency (NUE) are desirable.
Activation tagging can be utilized to identify genes with the ability to affect a trait. This approach has been used in the model plant species Arabidopsis thaliana (Weigel et al., Plant Physiol. 122:1003-1013 (2000)). Insertions of transcriptional enhancer elements can dominantly activate and/or elevate the expression of nearby endogenous genes. This method can be used to identify genes of interest for a particular trait (e.g. nitrogen use efficiency in a plant, drought tolerance in plant), genes that when placed in an organism as a transgene, can alter that trait.
OsNAC3/ONAC067 is a NAC transcription factor which appears to be widespread in plants. Extensive investigation aided by the availability of several complete plant genomic sequences has identified 117 NAC genes in Arabidopsis, 151 in rice (Nuruzzaman et al., frontier in microbiology. 248:1-16 (2013)). The functions of this gene family are being studied broadly.